1. Field of the Invention
The invention is directed to the field of continuous embossing of sheeting or webs and more particularly to methods and apparatus of producing large scale, flexible, and generally cylindrical embossing tools to emboss continuous plastic webs or the like with a highly accurate pattern of cube-corners useful in the manufacture of retroreflective sheeting.
2. Description of the Prior Art
Some presently employed techniques for the production of retroreflective sheeting include the casting of cube corners on cylindrical drums, followed by an application of secondary material, whereby the cube corner elements are adhered to a different back-up material. (e.g., U.S. Pat. No. 3,935,359).
Sequential embossing of cube corner type sheeting material has been suggested by using a series of tooled plates and molds. The web of material is embossed on one stroke of the press and then indexed to the next station for a further pressing operation (U.S. Pat. No. 4,244,683). This process, while operating on a continuous strip of material, is only sequential in nature and has all of the economic and manufacturing deficiencies inherent in such a process. Moreover, to be economically feasible, the width of film or sheeting to be produced, such as 48", requires extensive mold handling capability not contemplated by the Rowland '683 structure and process.
Small, rigid cylindrical rolls also are available for the general continuous embossing of webs of sheet material but, because of the high degree of optical accuracy required in reproducing cube corner elements, this technique has not been used to produce continuous sheeting.
Continuous belt type embossing tools also have been disclosed for embossing non-optically critical patterns in thermoplastic or other materials, such as shown in Bussey et. al., U.S. Pat. No. 3,966,383. It also is well known in the cube-corner reflector art to use electroformed tools for producing mold elements. However, the relatively small area encompassed by the typical reflective area permits the easy separation of the electroformed part from its "master" or from pins. To produce a tool required to emboss large areas of sheet, it would be possible to assemble larger and longer groups of masters, but minute seams would be found at the junction lines. Those seams in a final tool could produce stress risers, flash or fins, leaving the assembled tool with possible fatigue areas. In accordance with the present invention, the pieces are reproduced by eliminating the "fin" or seam and then by producing a cylindrical mother and electroforming internally of the tool mother. A problem then encountered is the removal of the cylindrical electroformed tool from the tool mother because of the very accurate but tightly interfitting male and female faces. The present invention discloses techniques and apparatus for producing a cylindrical embossing a tool by electroforms; and a method of separating the finished tool from the cylindrical tool mother.